This invention is generally directed to conductive composite particles and processes for the preparation thereof, and more specifically the present invention relates to small conductive polymeric composite particles, each comprising a polymer and a conductive filler distributed, preferably evenly throughout the polymer matrix of composite. The present invention also relates to processes for the preparation of polymeric composite particles. In one embodiment, the process of the present invention comprises the preparation of conductive polymeric particles containing a conductive filler distributed substantially throughout the polymer matrix of the particles, and which particles can be selected as carrier powder coatings. In another embodiment, the process of the present invention comprises the preparation of conductive polymeric composite particles with an average volume particle size diameter of from between about 0.05 micron to about 5 microns. The conductivity of the generated submicron polymeric composite particles can be modified by, for example, varying the weight percent of conductive filler component present in effective amounts of, for example, from between about 1 weight percent to about 50 weight percent, and also by varying the composition of the conductive filler component. Thus, in embodiments conductive submicron polymeric composite particles with a conductivity of from between about 10.sup.-10 (ohm-cm).sup.-1 to about 10.sup.-1 (ohm-cm).sup.-1 can be prepared. In embodiments, the particles with average diameter of about 0.05 to about 5 microns conductive composite particles are comprised of polymer and a conductive filler distributed evenly throughout the polymer matrix of the composite product, and which product can be obtained by a modified semisuspension polymerization method in which at least one monomer is mixed with a conductive filler, solvent, one or more polymerization initiators, and a chain transfer component; effecting solution polymerization by heating until from about 80 to about 100 weight percent of the monomer has been polymerized; drying the mixture by removing the solvent; dispersing the aforementioned mixture of conductive filler or fillers and polymer in at least one monomer with one or more polymerization initiators, a crosslinking agent and a chain transfer agent; dispersing the resulting mixture in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.05 to about 5 microns in water; polymerizing the resulting suspension by heating; and subsequently optionally the product is washed and dried.
Metals such as carrier cores are conductive or semiconductive materials, and the polymeric materials used to coat the surface of metals are usually insulating. Therefore, carrier particles coated completely with polymer or a mixture of polymers can lose their conductivity and become insulating. Although this is desired for some applications, for conductive magnetic brush systems (CMB) the carrier particles should be conductive. Since the carrier polymer coating can be utilized to control carrier tribo, a conductive carrier coating is needed to design carriers with the desired conductivity and triboelectrical properties. Conductive polymers are very costly, and are not suitable for preparing low cost, for example less than $5/pound, coating, thus a conductive polymer composite comprising a low cost polymer and a conductive filler, such as conductive carbon black, is considered a more suitable alternative.
A polymer composite coating of metal materials, such as carrier beads, is known and can generally be obtained by two general approaches, solution and powder coating. Solution coating of carriers with a polymer composite solution comprised of a polymer, a conductive filler and solvent can be utilized to prepare conductive carrier, however, trapping of solvent in the solution coating can adversely interfere with the use of coated materials, for example the residual solvent trapped in the carrier coating reduces the carrier life, and the release of solvent in the developer housing can cause other problems related to the harmful effects of absorbed solvent to various copying machine parts and the toxicity of solvent. Moreover, the solvent recovery operation involved in the solution coating processes is costly. The powder coating of metal surfaces can eliminate the need for solvent, and therefore, many of the problems associated with solution coating; however, it requires a polymer powder with a very small size, for example less than 1 to 5 microns. Although polymer powders are available for carrier powder coating, submicron or micron-sized polymer composite particles containing conductive filler to prepare conductive coated carriers that maintain their triboelectrical characteristics for extended time periods exceeding, for example, 200,000 images and which possess the other advantages illustrated herein are desired.
Semisuspension polymerization process is known, reference U.S. Pat. No. 5,236,629, the disclosure of which is totally incorporated herein by reference. The '629 patent describes a process for the preparation of conductive submicron polymeric particles which comprises mixing at least one monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 to about 50 weight percent of the monomer has been polymerized; terminating polymerization by cooling the partially polymerized monomer; adding thereto from about 1 to about 50 weight percent of a conductive filler, or conductive fillers, followed by mixing thereof; dispersing the aforementioned mixture of conductive filler or fillers, and partially polymerized product in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.05 to about 1 micron in water; polymerizing the resulting suspension by heating; and subsequently washing and drying the product. There are some disadvantages with the '629 process which can be overcome by the modified semisuspension polymerization process of the present invention. For example, although most carbon blacks, which are known to be free radical inhibitors, can be used in the '629 process, those carbon blacks which are very strong inhibitors, for example some highly oxidized carbon blacks, can inhibit the polymerization of the '629 process. In the modified semisuspension polymerization process of the present invention it is possible to select very strongly inhibiting carbon blacks. Furthermore, the nature of the conductive filler dispersion in the '629 process is one of free filler particles physically dispersed in the polymer matrix, while in the modified semisuspension polymerization process of the present invention the filler, such as carbon black, contains the polymer made in the solution polymerization step chemically grafted or strongly adsorbed onto its surface thereby enhancing its dispersion stability. Therefore, with the present invention there are fewer problems associated with free filler particles that can occur in the '629 process if the bulk polymerization step is not conducted to the optimum conversion. The modified semisuspension polymerization process of the present invention also provides advantages in obtaining a more uniform dispersion of filler in the final particles primarily because of the grafted or adsorbed polymer on the filler surface. While some additives are difficult to disperse in various monomers, depending on the surface compatibility of the filler and monomers, the modified semisuspension polymerization process overcomes this difficulty by chemically grafting or adsorbing polymer on the filler surface during the solution polymerization. This solution polymerization step, therefore, compatibilizes the filler with the monomer, thereby ensuring better filler dispersion than in the '629 process. This results in fewer final particles without filler, which is particularly important for conductive particles, as there can be a significant decrease in conductivity when even a small fraction of nonconductive particles, that is particles without filler, are mixed in with the conductive particles.
The preparation of polymeric particles for powder coatings can be accomplished, for example, by three methods, namely grinding or attrition, precipitation and in situ particle polymerization. Grinding or attrition, especially fluid energy milling, of large polymeric particles or polymeric composite particles containing fillers to the size needed for powder coating, for example less than 1 to 5 microns, is often not desirable both from an economic and functional viewpoint. These materials are difficult to grind and, therefore, grinding or attrition of required materials for coating with present milling equipment is very costly due to very low processing yield, for example in the range of 5 to 10 weight percent. Precipitation process can also be used to prepare polymeric/polymeric composite particles. In one process, the polymer solution is heated to above its melting temperature and then cooled to form particles. In another process, the polymer solution is precipitated using a nonsolvent or the polymer solution is spray dried to obtain polymeric/polymeric composite particles. With all these precipitation processes, it has been difficult to achieve low cost, pure polymer, that is, for example, with no or substantially no impurities such as solvents or precipitants in the resulting polymer particles. It is also difficult to obtain particles with small particle size and narrow particle size distribution. It is also difficult to control filler distribution throughout each particle's polymer matrix. In the in situ particle polymerization process, polymer particles are prepared by using suspension dispersion, emulsion and semisuspension polymerization. Suspension polymerization can be utilized to prepare polymer particles and polymeric composite particles containing, for example, a conductive filler. However, this process does not, for example, effectively enable particles with a size of less than 5 microns. Although emulsion and dispersion polymerization can be utilized to prepare polymeric particles of a small size, for example less than 5 microns, these processes wherein particle formation is achieved by nucleation and growth do not, it is believed, enable synthesis of particles containing fillers such as conductive fillers.
There is disclosed in U.S. Pat. No. 4,908,665 a developing roller or developer carrier comprised of a core shaft, a rubber layer and a resin coating layer on the surface of the rubber containing conductive fillers for a one component developer. It is indicated in the '665 patent that a conductive developing roller can eliminate variation of the image characteristic due to the absorption of moisture for one component development. This patent thus describes a developing roller for one component developer. U.S. Pat. No. 4,590,141 discloses carrier particles for two component developer coated with a layer of silicon polymer using fluidized bed solution coating. U.S. Pat. No. 4,562,136 discloses a two component dry type developer of carrier particles coated with a silicon resin containing a monoazo metal complex charging. The two component carriers described in the above two patents are insulating and are not believed to be conductive. There is disclosed in U.S. Pat. No. 4,912,005 a conductive carrier composition coated with a layer of resin containing a conductive particle by solution coating. Residual solvent trapped in the aforementioned coated layer adversely effects the maintainability of carrier electrical properties for an extended time period.
There is disclosed in U.S. Pat. No. 3,505,434 a process wherein particles for fluidized bed powder coating are prepared by dispersing the polymer in a liquid which is heated to above the polymer melting point and stirred causing the polymer particles to form. The particles are then cooled below their melting point and recovered. However, this process does not, for example, enable particles with a size of below 50 microns in average volume diameter.
Also, the suspension polymerization of monomer is known for the formation of polymer/polymeric composite particles generally in a size range of about 200 microns and higher. The main advantage of suspension polymerization is that the product may easily be recovered, therefore, such a process is considered economical. However, it is very difficult by suspension polymerization to prepare very small particles as the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky. For example, there is disclosed in U.S. Pat. No. 3,243,419 a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in the coalescence of the particles. Also disclosed in U.S. Pat. No. 4,071,670 is a method of suspension polymerization wherein the monomer initiator mixture is dispersed in water containing stabilizer by a high shear homogenizer, followed by polymerization of suspended monomer droplets.
Further, disclosed in U.S. Pat. No. 4,835,084 is a method for preparing pigmented particles wherein high concentration of silica powder is used in the aqueous phase to prevent coalescence of the particles. There is also disclosed in U.S. Pat. No. 4,833,060 a process for the preparation of pigmented particles by dissolving polymer in monomer and dispersing in the aqueous phase containing silica powder to prevent coalescence of the particles. However, the silica powder used in both U.S. Pat. Nos. -084 and -060 should be removed using KOH which is costly, and residual KOH and silica materials remaining on the surface adversely affects the charging properties of particles. There is also disclosed in U.S. Pat. No. 3,954,898 a two step polymerization process for the preparation of a thermosetting finished powder. However, this process does not enable synthesis of particles with a size less than about 100 microns. Moreover, this patent does not teach the synthesis of submicron particles containing conductive fillers.
As a result of a patentability search for U.S. Pat. No. 5,043,404 (D/89032), there were located U.S. Pat. No. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; and 4,680,200 and 4,702,988, which illustrate emulsion polymerization. It is known that submicron polymeric particles can be synthesized by emulsion polymerization. However, synthesis of submicron polymeric particles by emulsion polymerization requires a high concentration of emulsifier which remains in the final product and renders it humidity sensitive. Therefore, emulsion polymerization does not enable preparation of clean submicron polymeric particles which are not sensitive to humidity. Moreover, in the emulsion polymerization particle formation is controlled by diffusion of monomer from monomer droplet through a water phase into the growing particles. This mechanism, which is characteristic of emulsion polymerization, does not allow inclusion of conductive fillers in the polymeric particles. Furthermore, it is known that the addition of conductive fillers into emulsion, dispersion or suspension polymerization systems causes severe inhibition which stops or reduces the rate of polymerization significantly.
Disclosed in the aforementioned U.S. Pat. No. 5,043,404 (D/89032), the disclosure of which is totally incorporated herein by reference, is a semisuspension polymerization process for the preparation of small polymeric particles which are comprised of a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component which are bulk polymerized until partial polymerization is accomplished. The resulting partially polymerized monomer or comonomers is dispersed in water containing a stabilizer component with, for example, a high shear mixer, then the resulting suspension polymerized, followed by washing and drying the submicron polymeric particles. U.S. Pat. No. 5,236,629 discloses a process for the preparation of conductive submicron polymeric particles which comprises mixing at least one monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 to about 50 weight percent of the monomer has been polymerized; terminating polymerization by cooling the partially polymerized monomer; adding thereto from about 1 to about 50 weight percent of a conductive filler, or conductive fillers, followed by mixing thereof; dispersing the aforementioned mixture of conductive filler or fillers, and partially polymerized product in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.05 to about 1 micron in water; polymerizing the resulting suspension by heating; and subsequently washing and drying the product.
The modified semisuspension polymerization process described in the present application offers a different process with significant improvements over the process disclosed in U.S. Pat. No. 5,236,629. These advantages include (1) superior uniformity of the conductive filler in the final particles because of the solution polymerization step in which polymer is grafted or adsorbed onto the surface of the filler; (2) fewer particles containing no conductive filler due to improved dispersion; (3) higher conductivity in the final particles at the same weight percent loading of conductive filler because of the better filler dispersion; (4) fewer potential problems with free conductive filler particles contaminating the final product and causing, for example, problems with contamination in the developer housing when the particles are coated on carrier cores; and (5) the ability to use fillers which are very strong free radical polymerization inhibitors such as some highly oxidized carbon blacks. With the present invention, a process to obtain conductive submicron or micron-sized polymer particles of less than about 1 to about 5 microns in average volume diameter as determined by a scanning electron microscope, each containing conductive fillers evenly dispersed in the polymer. These polymeric particles contain from about 1 to about 50 weight percent of a conductive filler, such as carbon black, which is evenly distributed throughout the polymer matrix. This modified semisuspension polymerization process permits the preparation of low cost, clean, and dry submicron conductive polymeric particles that can be selected as carrier powder coatings.